• Caspar Hare, associate professor in the Department of Linguistics and Philosophy

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How we (should) decide

Philosopher aims to develop theories of practical rationality that may just help us make real-world decisions.


Caspar Hare is interested in your choices. Not the ones you’ve already made, but the ones you will make, and how you’ll go about making them. The more important, the better.

By way of example, suppose you’re deciding between two careers: journalism and physics. You enjoy both, but for different reasons: Journalism lets you interact with a broad swath of society, exercise your passion for writing and reach a wider audience; physics, though, represents the allure of science, with the freedom to chart a research trajectory at the forefront of human knowledge.

Suppose, too, for argument’s sake, that you had a pretty good idea of how each career would turn out. Either way, you’d be successful and recognized within your field. You’d live in a desirable location and make a good salary.

In your mind, the two options — call them J and P — are so equally and oppositely attractive that you truly cannot decide. But now suppose someone threw a third option into the mix: another journalism career, J*, identical to the first but paying an extra $50 a year. You probably prefer J* to J — why not? But do you prefer J* to P?

If you’re like most people, the answer is “not really.” Fifty dollars a year is not enough to sway you between two choices that are so radically different. And yet this outcome poses a big problem for traditional theories of rationality. Hare, a newly tenured associate professor of philosophy at MIT, studies this problem, which is known to philosophers as “negative intransitivity.” And he’s had to make some career choices of his own along the way.

Incommensurate values

To understand negative intransitivity, first recall the transitive property: If you prefer A to B and B to C, then you prefer A to C.

Preferences that are transitive, Hare says, should also be negatively transitive, meaning that “if you’re indifferent between A and B and indifferent between B and C, you should also be indifferent between A and C.” But that’s not the case in the above example: Most people say they’re indifferent between J and P and also between P and J*, but they prefer J* to J. Does that make them irrational?

Not necessarily, Hare says; it just means that we need to augment our ideas about rationality.

“I’m trying to expand the theory of practical rationality so that it applies to people whose preferences fall into that structure,” he says. “When things like money are at stake, it’s fairly easy to represent preferences with numbers. But if other things are at stake, it’s not so easy. It’s particularly hard when the two things exemplify really different kinds of values — when they’re good in really different ways.”

Hare thinks the key is to use not just single functions to represent preferences, but sets of functions, adding dimensions of complexity that will allow for multiple levels of comparison. Then, choices could be ranked based on the outcomes of all the functions in the set of functions that represent them. Though the modeling can get abstract, Hare says the focus is ultimately on applying the model to practical, real-world scenarios.

“Given that you have certain desires and certain beliefs, the idea that is you could use this [model] to tell you what you ought rationally to do in a given situation,” Hare says.

Coming around to philosophy

So where did “philosophy professor” rank on Hare’s own list of career choices? Hare, who grew up in London, says for the first half of his life, it didn’t even occur to him.

“I was not a very academically inclined person at all. [As a teenager] I’d started reading some philosophy, but I never really associated it as something you did in school. In fact, it always seemed like the anti-school,” Hare says, adding that he attended a “rigid” boarding school that focused on tests and rote memorization.

“Philosophy seemed incredibly anarchic because you got to question the fundamental assumptions of all these disciplines,” Hare continues. Still, he says it never crossed his mind to make philosophy “a serious academic pursuit.”

Video: Melanie Gonick

Even while attending Wesleyan University in Connecticut, Hare majored in intellectual history, but didn’t necessarily focus on philosophy. After a brief stint back in England working on the business end of the Financial Times, Hare, now 39, realized that he could turn his side interest into a career.

He returned to the United States to earn a master’s degree at Stanford University and a PhD at Princeton University, tackling problems in normative ethics and metaphysics. Hare’s first book, On Myself, And Other, Less Important Subjects (Princeton University Press, 2009), was a partial revival of the theory of solipsism, in which he claims that the fact that one’s own self has a special status in the world need not preclude us from making sound moral judgments involving others.

‘There’s nothing you can’t think’

Indeed, it’s this ability to reason about morality, instead of simply relying on emotional gut reactions, that Hare considers one of philosophy’s greatest offerings to the next generation.

“What people, and young people in particular, think about moral questions is powerfully influenced by emotional responses that they have — in particular, disgust-related emotional responses, which are acquired via socialization,” Hare says. “It’s good for people to be able to step back and think about how to respond to a moralized case not by just saying, ‘How do I immediately feel about this? Does it set off my ‘yuck response’?’ but knowing how to think carefully about it and really evaluate what’s going on.”

At MIT, Hare enjoys teaching and working with students. Quite a few undergraduates take at least one philosophy class during their time at the Institute, he says, which he believes helps them learn to think in a “disciplined way,” no matter what their career path.

“In philosophy, there’s nothing you can’t think,” Hare says. “Everything’s on the table, and the values are all about rigor and clarity, exploring how to use a thought and seeing where it goes.”


Topics: Books and authors, Education, teaching, academics, Faculty, Humanities, Philosophy

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